The manufacture of an integrated circuit in a semiconductor device involves the formation of a metal layer that typically contains a wiring pattern which is overlaid on another conductive pattern. This process is repeated several times to produce a stack of metal layers. Metal interconnects which form horizontal and vertical electrical pathways in the device are separated by insulating or dielectric materials to prevent crosstalk between the metal wiring that can degrade device performance by slowing circuit speed. A popular method of forming an interconnect structure is a dual damascene process in which vias and trenches are filled with metal in the same step. A single damascene process is also commonly employed to form a metal pattern in one or more dielectric layers. The most frequently used dual damascene approach is a via first process in which a via is formed in a stack of dielectric layers and then a trench is formed above the via. Recent improvements in dual damascene processing include lowering the resistivity of the metal interconnect by switching from aluminum to copper and reducing the dielectric constant (k) of insulating materials to avoid capacitance coupling between the metal interconnects.
Current manufacturing practices involve forming vias and trenches that have sub-micron dimensions which can be less than 0.25 microns in width. One of the more promising low k dielectric materials is organosilicate glass (OSG) also known as SiCOH which is a silicon oxide that is doped with carbon and hydrogen atoms. Silicon oxide which has been traditionally used as a dielectric material has a dielectric constant of about 4. SiCOH has a k value between about 2 and 3 and thereby provides a much needed reduction in capacitance coupling between wiring. SiCOH is available as Black Diamond™ from Applied Materials, CORAL™ from Novellus, or can be obtained by different trade names from other manufacturers. The composition and properties of SiCOH may vary depending on the deposition conditions and source gases.
One concern with using SiCOH in a damascene structure is that the material as deposited is porous. A porous structure will allow moisture uptake which increases the dielectric constant and defeats the purpose of depositing a low k dielectric material. An organosilicate glass layer is employed as a thick dielectric layer in U.S. Pat. No. 6,472,333. A SiC cap layer is formed on the organosilicate glass (SiCOH) layer to provide increased hardness for a subsequent chemical mechanical polish (CMP) step and then the SiCOH layer is annealed for improved mechanical properties and a lower k value. An amorphous carbon cap layer on a low k dielectric layer is described in U.S. Pat. No. 6,541,397 and serves as an etch mask and as a CMP stop layer.
In some cases, densification after annealing is desirable. A well known method of densifying a porous SiCOH layer is to perform a plasma treatment such as the N2/NH3 plasma process described in U.S. Pat. No. 6,436,808. Besides stabilizing the dielectric constant, the densification also improves SiCOH resistance to etchants such as O2 plasma during removal of a photoresist mask that is used to transfer a trench pattern into the damascene stack.
The integration of amorphous silicon carbide (α-SiC:H) as a barrier/etch stop layer in a copper damascene fabrication scheme has been suggested as a possible solution to the problems of parasitic capacitance and RC delay in ultra-large scale integration. Although the α-SiC:H film has a lower dielectric constant (k˜4.5) than silicon nitride (k˜7), α-SiC:H has a higher current leakage level under high bias and a lower breakdown field than silicon nitride. Nitrogen doped SiC (SiCN) has been used as a barrier layer in a damascene structure as mentioned in U.S. Pat. No. 6,436,824. While SiCN can improve the leakage performance, trace amounts of amines in SiCN have a tendency to poison a photoresist layer in a via hole during patterning of a trench opening in a via first dual damascene scheme. As a result, photoresist residue remains in the via after exposed regions are developed in an aqueous base solution which leads to an expensive rework process. In addition, the dielectric constant of SiCN (k˜4.9) is higher than the desired value of less than 4 and preferably less than 3 for a low k dielectric material. Therefore, an improved barrier layer or etch stop layer is required for new technologies which has a higher breakdown field and lower dielectric constant than current materials and which does not contain nitrogen that can have a deleterious effect on photoresist patterning.
One prior art method that mitigates the poisoning effect of a SiCN etch stop layer is described in U.S. Pat. No. 6,455,417 where a composite etch stop comprised of an upper carbon doped oxide (SiCOH) is formed over a lower SiCN layer on a substrate. The lower layer acts as a buffer to keep the oxide layer from oxidizing the underlying conductive metal while the SiCOH layer prevents the photoresist poisoning issue. However, this prior art does not address the issue of a relatively high k value for SiCN and a thick SiCOH layer may be necessary to prevent amines in SiCN from diffusing through the porous upper layer.
Other low k dielectric materials such as benzocyclobutene or hydrogen silsesquioxane (HSQ) are employed as an etch stop layer in a damascene structure in U.S. Pat. No. 6,417,090. However, there is no provision to form a buffer layer between the oxygen containing HSQ layer and an underlying copper pattern.
A carbon doped silicon oxide layer is formed on a substrate in U.S. Pat. No. 6,410,462 and uses silane, an oxygen source, and a mixture of CH4 and acetylene for the deposition step. The introduction of methane and acetylene into the CVD process is claimed to promote a lower film density by forming more Si—O network terminating species. In this case, the composition of the SiCOH film appears to be less crosslinked than is normally desired and may result in a less mechanically sturdy structure. Low density also implies a higher porosity that can lead to water absorption and higher k value in subsequent processing steps.
An oxygen or nitrogen doped SiC layer is employed as an etch stop layer in U.S. Pat. No. 6,486,082. However, the concentration of the dopant is not described.
A method of incorporating a SiCOH layer with a low oxygen content, hereafter referred to as oxygen doped silicon carbide, as an etch stop or barrier layer in a dual damascene scheme is desirable so that a reduction in dielectric constant and a higher breakdown field can be achieved without compromising Cu barrier capability or a photoresist processing step. An oxygen doped silicon carbide etch stop layer should have good etch selectivity to other low k dielectric layers including SiCOH layers like Black Diamond™ from Applied Materials or CORAL™ available from Novellus.